The ratio of the banner is 5:2. I is a vertical blue-white bicolour with the coat of arms shifted to the top.
Source: Reppenstedt municipal webpage, showing the banner together with the German and European banner.
Klaus-Michael Schneider, 4 Oct 2007

It is a green over yellow horizontal bicolour with centred arms.
Sources:
(1) this online catalogue(2) §2(2) of Hauptsatzung of Reppenstedt Municipality, version 18 March 2002
Klaus-Michael Schneider, 23 July 2017